Shih-Yuin Lin

Entanglement Dynamics between Inertial and Non-uniformly Accelerated Detectors

A bstractWe study the time-dependence of quantum entanglement between two Unruh-DeWitt detectors, one at rest in a Minkowski frame, the other non-uniformly accelerated in some specified way. The two detectors each couple to a scalar quantum field but do not interact directly. The primary challenge in problems involving non-uniformly accelerated detectors arises from the fact that an event horizon is absent and the Unruh temperature is ill-defined. By numerical calculation we demonstrate that the correlators of the accelerated detector in the weak coupling limit behaves like those of an oscillator in a bath of time-varying “temperature” proportional to the instantaneous proper acceleration of the detector, with oscillatory modifications due to non-adiabatic effects. We find that in this setup the acceleration of the detector in effect slows down the disentanglement process in Minkowski time due to the time dilation in that moving detector.

Relativistic Quantum Information in Detectors-Field Interactions

We review Unruh–DeWitt detectors and other models of detector–field interaction in a relativistic quantum field theory setting as a tool for extracting detector–detector, field–field and detector–field correlation functions of interest in quantum information science, from entanglement dynamics to quantum teleportation. In particular, we highlight the contrast between the results obtained from linear perturbation theory which can be justified provided switching effects are properly accounted for, and the nonperturbative effects from available analytic expressions which incorporate the backreaction effects of the quantum field on the detector behavior.

Unruh effect under non-equilibrium conditions: oscillatory motion of an Unruh-DeWitt detector

A bstractThe Unruh effect refers to the thermal fluctuations a detector experiences while undergoing linear motion with uniform acceleration in a Minkowski vacuum. This thermality can be demonstrated by tracing the vacuum state of the field over the modes beyond the accelerated detector’s event horizon. However, the event horizon is well-defined only if the detector moves with eternal uniform linear acceleration. This idealized condition cannot be fulfilled in realistic situations when the motion unavoidably involves periods of non-uniform acceleration. Many experimental proposals to test the Unruh effect are of this nature. Often circular or oscillatory motion, which lacks an obvious geometric description, is considered in such proposals. The proper perspective for theoretically going beyond, or experimentally testing, the Unruh-Hawking effect in these more general conditions has to be offered by concepts and techniques in non-equilibrium quantum field theory. In this paper we provide a detailed analysis of how an Unruh-DeWitt detector undergoing oscillatory motion responds to the fluctuations of a quantum field. Numerical results for the late-time temperatures of the oscillating detector are presented. We comment on the digressions of these results from what one would obtain from a naive application of Unruh’s result.

Quantum entanglement and entropy in particle creation

We investigate the basic theoretical issues in the quantum entanglement of particle pairs created from the vacuum in a time-dependent background field or spacetime. Similar to entropy generation from these processes which depends on the choice of physical variables and how certain information is coarse grained, entanglement dynamics hinges on the choice of measurable quantities and how the two parties are selected as well as the background dynamics of the field or spacetime. We discuss the conditions of separability of quantum states in particle creation processes and point out the differences in how the von Neumann entropy is used as a measure of entropy generation versus for entanglement dynamics. We show by an explicit construction that adoption of a different set of physical variables yields a different entanglement entropy. As an application of these theoretical considerations we show how the particle number and the quantum phase enter the entanglement dynamics in cosmological particle production.

Quantum teleportation between moving detectors

We consider the quantum teleportation of continuous variables modeled by Unruh-DeWitt detectors coupled to a common quantum field initially in the Minkowski vacuum. An unknown coherent state of an Unruh-DeWitt detector is teleported from one inertial agent (Alice) to an almost uniformly accelerated agent (Rob, for relativistic motion), using a detector pair initially entangled and shared by these two agents. The averaged physical fidelity of quantum teleportation, which is independent of the observers frame, always drops below the best fidelity value from classical teleportation before the detector pair becomes disentangled with the measure of entanglement evaluated around the future lightcone of the joint measurement event by Alice. The distortion of the quantum state of the entangled detector pair from the initial state can suppress the fidelity significantly even when the detectors are still strongly entangled around the lightcone. We point out that the dynamics of entanglement of the detector pair observed in Minkowski frame or in quasi-Rindler frame are not directly related to the physical fidelity of quantum teleportation in our setup. These results are useful as a guide to making judicious choices of states and parameter ranges and estimation of the efficiency of quantum teleportation in relativistic quantum systems under environmental influences.

Entanglement, recoherence and information flow in an accelerated detector-quantum field system : implications for the black hole information issue

We study an exactly solvable model where a uniformly accelerated detector is linearly coupled to a massless scalar field initially in the Minkowski vacuum. Using the exact correlation functions obtained in Lin and Hu (2006 Phys. Rev. D 73 124018, 2007 Phys. Rev. D 76 064008) we show that as soon as the coupling is switched on one can see information flowing from the detector to the field and propagating with the radiation into null infinity. By expressing the reduced density matrix of the detector in terms of the two-point functions, we calculate the purity function in the detector and study the evolution of quantum entanglement between the detector and the field. Only in the ultraweak coupling regime could some degree of recoherence in the detector appear at late times, but never in full restoration. We explicitly show that under the most general conditions the detector never recovers its quantum coherence and the entanglement between the detector and the field remains large at late times. To the extent that this model can be used as an analog to the system of a black hole interacting with a quantum field, our result seems to suggest in the prevalent non-Markovian regime, assuming unitarity for the combined system, that black hole information is not lost but transferred to the quantum field degrees of freedom. Our combined system will evolve into a highly entangled state between a remnant of large area (in Bekensteins black hole atom analog) without any information of its initial state, and the quantum field, now imbued with complex information content not-so-easily retrievable by a local observer.

Boundary effects on quantum entanglement and its dynamics in a detector-field system

A bstractIn this paper we analyze an exactly solvable model consisting of an inertial Unruh-DeWitt detector which interacts linearly with a massless quantum field in Minkowski spacetime with a perfectly reflecting flat plane boundary. Firstly a set of coupled equations for the detector’s and the field’s Heisenberg operators are derived. Then we introduce the linear entropy as a measure of entanglement between the detector and the quantum field under mirror reflection, and solve the early-time detector-field entanglement dynamics. After coarse-graining the field, the dynamics of the detector’s internal degree of freedom is described by a quantum Langevin equation, where the dissipation and noise kernels respectively correspond to the retarded Green’s functions and Hadamard elementary functions of the free quantum field in a half space. At late times when the combined system is in a stationary state, we obtain exact expressions for the detector’s covariance matrix and show that the detector-field entanglement decreases for smaller separation between the detector and the mirror. We explain the behavior of detector-field entanglement qualitatively with the help of a detector’s mirror image, compare them with the case of two real detectors and explain the differences.

Entanglement dynamics of detectors in an Einstein cylinder

A bstractWe investigate how nontrivial topology affects the entanglement dynamics between a detector and a quantum field and between two detectors mediated by a quantum field. Nontrivial topology refers to both that of the base space and that of the bundle. Using a derivative-coupling Unruh-DeWitt-like detector model interacting with a quantum scalar field in an Einstein cylinder S1 (space) × R1 (time), we see the beating behaviors in the dynamics of the detector-field entanglement and the detector-detector entanglement, which distinguish from the results in the non-compact (1+1) dimensional Minkowski space. The beat patterns of entanglement dynamics in a normal and a twisted field with the same parameter values are different because of the difference in the spectrum of the field modes. In terms of the kinetic momentum of the detectors, we find that the contribution by the zero mode in a normal field to entanglement dynamics has no qualitative difference from those by the nonzero modes.

Quantum radiation by an Unruh-DeWitt detector in oscillatory motion

A bstractQuantum radiated power emitted by an Unruh-DeWitt (UD) detector in linear oscillatory motion in (3+1)D Minkowski space, with the internal harmonic oscillator minimally coupled to a massless scalar field, is obtained non-perturbatively by numerical method. The signal of the Unruh-like effect experienced by the detector is found to be pronounced in quantum radiation in the highly non-equilibrium regime with high averaged acceleration and short oscillatory cycle, and the signal would be greatly suppressed by quantum interference when the averaged proper acceleration is sufficiently low. An observer at a fixed angle would see periods of negative radiated power in each cycle of motion, while the averaged radiated power over a cycle is always positive as guaranteed by the quantum inequalities. Coherent high harmonic generation and down conversion are identified in the detector’s quantum radiation. Due to the overwhelming largeness of the vacuum correlators of the free field, the asymptotic reduced state of the harmonics of the radiation field is approximately a direct product of the squeezed thermal states.